refactor(netxlite): allow easy dialer chain customization (#770)

This diff modifies the construction of a dialer to allow one
to insert custom dialer wrappers into the dialers chain.

The point of the chain in which we allow custom wrappers is the
optimal one for connect, read, and write measurements.

This new design is better than the previous netx design since
we don't need to construct the whole chain manually now.

The work in this diff is part of the effort to make engine/netx
just a tiny wrapper around netxlite.

See https://github.com/ooni/probe/issues/2121.

internal/netxlite/quic_test.go

@@ -30,7 +30,7 @@ func TestNewQUICDialer(t *testing.T) {
 		t.Fatal("invalid logger")
 	}
 	resolver := logger.Dialer.(*quicDialerResolver)
-	if _, okay := resolver.Resolver.(*nullResolver); !okay {
+	if _, okay := resolver.Resolver.(*NullResolver); !okay {
 		t.Fatal("invalid resolver type")
 	}
 	logger = resolver.Dialer.(*quicDialerLogger)